A survey by the Pew Internet and American LIfe Project described in Usability News – Social Networking Sites used by More than Half US Teenagers Online shows that 55% of all online American youths ages 12-17 use online social networking sites.
The survey also finds that older teens, particularly girls, are more likely to use these sites. For girls, social networking sites are primarily places to reinforce pre-existing friendships; for boys, the networks also provide opportunities for flirting and making new friends.
More key findings are available in the original article.